1. Field of the Invention
The present invention relates to novel liquid dielectric compositions and, more especially, to liquid dielectrics comprising a mixture of a mixed phthalate of isobutyl alcohol and a branched higher alkanol of a diisobutyl phthalate and/or a phthalate of a branched higher alkanol.
2. Description of the Prior Art
The prior art recognizes various dielectric materials to be used, for example, as insulation for electrical equipment. It is appreciated that the liquids employed as these insulating materials in various electrical apparatus optimally combine certain properties such as, for example, a high permittivity of generally between about 4.8 and 6.0 at 90.degree. C., a low coefficient of dissipation, good resistance to decomposition under the influence of diverse factors existing in service of the apparatus, a low viscosity at low temperatures, a low solidification point, and a low susceptibility to crystallization at the temperatures encountered during service. Moreover, while it is possible to balance these properties in a liquid dielectric composition when first formulated, obviously it is to be desired that these same properties maintain optimum values for the full duration of the life of the device or apparatus in which they are employed (e.g., condensers, transformers, etc.), regardless of the severity of use to which the devices are subjected and in spite of the length of service life projected therefor. Accordingly, it is manifest that the liquid dielectrics should exhibit substantial stability of the properties with time, which stability depends essentially on the stability of the chemical compositions utilized. Another complicating factor which materially effects a lack of stability in prior art liquid dielectric compositions regards the presence of various impurities, whether organic or inorganic in origin, which are inherent in the composition through, for example, formulation methodology, or as may be produced during the life of the device wherein the composition is utilized under service conditions including, most notably, temperature, pressure, and electrical voltage.
Various dielectric compositions and mixtures thereof have been proposed including, by way of example, mineral oils, aromatic chlorinated derivatives and certain esters of aliphatic or aromatic monoacids or polyacids. Typically, these compositions exhibit all of the requisite properties enumerated above, but to varying degrees which are rarely optimum and typically are balanced one against another to provide a serviceable dielectric.
Among these dielectric compositions found useful heretofore, it has been ascertained that the chlorinated polyphenyls (diphenyls and terphenyls), and in particular the chlorinated diphenyls, form a particularly valuable class of dielectric liquids in light of the ability of these compounds to combine many of the salient, and required, properties necessary for use as a dielectric liquid, while also possessed of suitable stability during service. Indeed, these compounds have been employed as liquid dielectrics essentially to the exclusion of all others for condenser applications. However, their use is not attended without disadvantages, particularly in light of their low biodegradability which fosters ecological problems. Accordingly, the art has recognized the need to develop products which are capable of adequately replacing this class of compounds.
Attention has been directed to the replacement of chlorinated polyphenyls by esters. This class of compositions again, however, presents serious disadvantages, particularly circumscribed by a pronounced lack of chemical stability under service conditions. Most particularly, the esters are sensitive to hydrolysis under the influence of even trace amounts of esterification catalysts, indigenous to the compositions unless one undertakes a difficult and expensive separatory process for the elimination of the same.
However, it has been found that the esters derived from alkanols, which are branched at the carbon in the .alpha.-position relative to the carbon which bears the alcohol group, possess remarkable stability to hydrolysis and, thus, are particularly suitable for use as dielectric liquids. In this regard, see U.S. Pat. No. 3,740,625, and Rutkowski et al, Conference IEEE Power Engineering Society, Jan. 30, 1975, Doc. C 75, 241-5, at 7-12. Specifically, the phthalates of alcohols branched in the .alpha.-position have been the subject of particular attention. Among the various compounds within this class may be mentioned diisobutyl phthalate, di-2-ethylhexyl phthalate, and diamyl phthalate. More particularly, diisobutyl phthalate and the mixed phthalates of isobutyl alcohol and higher alkanols branched in the .alpha.-position are of considerable interest due to their high permittivity: diisobutyl phthalate exhibiting a permittivity of 5.5 at 90.degree. C., and isobutyl-2-ethylhexyl phthalate a permittivity of 4.9 at 90.degree. C.
Notwithstanding these high permittivity values, diisobutyl phthalate and the mixed phthalates of isobutyl alcohol and branched alkanols may not be used alone. In fact, it has been ascertained (and while not hitherto reported in the prior art) that pure diisobutyl phthalate has a melting point of -8.degree. C., which results in crystallization of the composition at much too high a temperature to render the same useful within the context of the present invention. Other problems are evident with respect to the mixed isobutyl phthalates including a substantial difficulty in obtaining the same in a pure state due to the fact that during the reaction of phthalic acid or phthalic anhydride with isobutyl alcohol and the branched alkanol, diisobutyl phthalate and the phthalate of the branched alkanol are concomitantly formed, these latter components being very difficult and expensive to remove.
Consequently, it is essential to employ an ester mixture. However, it is known that ester mixtures can lead to inter-esterification reactions (alternately termed "redistribution reactions"), which result in the formation of a new mixture under the influences of temperature and impurities introduced by the esters themselves (esterification catalyst residues), or indeed formed from the metal parts (copper, tin, iron, and the like) of the device in which the dielectric is employed and with which the dielectric is in contact. This inter-esterification reaction can be represented, schematically, by: ##STR1##
As regards the utility of such a composition as a liquid dielectric, the reaction, which evinces a change in the overall chemical composition, runs counter to the essential requirement of stability of the various properties necessary to yield a satisfactory insulating component. The compositional variation may take place rapidly, under the influence of an abrupt rise in temperature of the apparatus or device in which the dielectric is contained, or it may take place gradually. In either event, there is present the danger of variation in the overall characteristics of the apparatus or device in which the liquid dielectric is employed, therefore restricting the utility of such a mixture of phthalates.
Consequently, the need exists to provide an insulating composition, particularly a liquid dielectric, for use in electrical apparatus and devices, which composition exhibits substantial stability and meets all of the aforementioned criteria for satisfactory dielectric materials. The need further exists to provide such a liquid dielectric based upon mixtures of esters which meet the enumerated criteria.